Correspondence to: Prof. HOU Shu-xun, Department of Orthopedics, First Affiliated Hospital of the General Hospital of the People's Liberation Army, Beijing, 100048, China (Tel: 86-10-66848863. Fax: 86-10-68989189 Email: userzyt@ sina.com.); ZHAO Yi-min, School of Stomatology, Fourth Military Medical University, Xi'an, Shaanxi 710032, China